Ultrasonic surgical instruments are useful surgical instruments for performing certain medical and surgical procedures. Generally, an ultrasonic surgical tool includes a handpiece that contains at least one piezoelectric driver. A tip is mechanically coupled to the driver and extends forward from the housing or shell in which the driver is disposed. The tip has a head. The head is provided with features, often teeth or flutes dimensioned to accomplish a specific medical/surgical task. An ultrasonic tool system also includes a control console. The control console supplies an AC drive signal to the driver. Upon the application of the drive signal to the driver, the driver cyclically expands and contracts. The expansion/contraction of the driver induces a like movement in the tip and, more particularly, the head of the tip. When the tip so moves, the tip is considered to be vibrating. The vibrating head of the tip is applied against tissue in order to perform a specific surgical or medical task. For example, some tip heads are applied against hard tissue. One form of hard tissue is bone. When this type of tip head is vibrated, the back and forth vibrations of the tip head remove, saw the adjacent hard tissue. Still other tip heads are designed to be placed against soft tissue. When this tip head vibrates the teeth often remove the tissue by a cutting action. Some ultrasonic tools also remove tissue by inducing cavitation in the tissue and surrounding fluid. Cavitation occurs as a result of the tip head moving back and forth. Specifically, as a result of these vibrations, small voids, cavities, form in the tissue and surrounding fluid. These cavities are very small zones of extremely low pressure. A pressure differential develops between contents of the cells forming the tissue and these cavities. Owing to the relatively large magnitude of this pressure differential, the cell walls burst. The bursting of these cell walls, removes, ablates, the cells forming the tissue.
The head of an ultrasonic tip is often relatively small. Some heads have diameters of less than 1.0 cm. An ultrasonic tool essentially only removes the tissue adjacent to where the head is applied. Thus owing to the relative small surface area of their heads, ultrasonic handpieces have proven to be useful tools for precisely removing both hard and soft tissue.
For an ultrasonic surgical instrument, sometimes called a handpiece or a tool, to efficiently function, a drive signal having the appropriate characteristics should be applied to the tool. If the drive signal does not have the appropriate characteristics, the tip head may undergo vibrations of less than optimal amplitude and/or may not vibrate as fast as possible. If the handpiece is in either state, the ability of the handpiece to, at a given instant, remove tissue may be appreciably reduced.
One means of ensuring an ultrasonic handpiece operates efficiently is to apply a drive signal to the handpiece that is at the resonant frequency of the handpiece. When the drive signal is at given voltage or current, the application of the drive signal at the resonant frequency induces vibrations in the tip that are at a relatively large amplitude in comparison to the application of the same voltage at a frequency that is off resonance.
Still other ultrasonic tool systems are designed to apply a drive signal at the anti-resonant frequency of the handpiece. The anti-resonant frequency may be a frequency at which the handpiece would have the highest impedance.
Applicant's SONOPET® Ultrasonic Aspirator includes a console with components designed to generate and apply a variable drive signal to the attached handpiece. Internal to the console is a resonance circuit. At the time of manufacture of the console, the inductance and capacitance of this resonance circuit are set as a function based on the impedance of the specific handpiece with which the console is intended to be used. The characteristics of the drive signal output by the console is set as a function of the voltage across this impedance circuit.
For many procedures, the SONOPET Console outputs a drive signal that at least is close to if not essentially identical to the resonant frequency of the mechanical components of the handpiece. However, in many normal use situations, an ultrasonic handpiece may be subjected to a significant mechanical load. This can happen, for example, when the tip is pressed against bone. In this situation, the mechanical load placed on the tip may cause a significant change in the impedance of the mechanical components of the handpiece. If this event occurs, the control console may not be able to output a drive signal at a frequency near the resonant frequency of the mechanical components of the handpiece.
Further, the impedance circuit internal to prior art console typically has an inductance and a capacitance that are set as a function of the specific handpiece with which the console is to be used. If a handpiece with different internal inductances, capacitances and resistances is attached to the console, there is an appreciable likelihood that the drive signal output by the console will not have the characteristics that facilitate the efficient operation of the handpiece. This makes it difficult, if not impossible, to use a console designed for use with one handpiece as the power supply to source a drive signal to another handpiece.